The present invention relates to a spark plug comprising a shell, a central electrode, an insulator surrounding the central electrode, said central electrode having a top end portion and a bottom end portion, and further comprising a ground electrode of which one of its end portions is secured to said shell and of which the other of its end portions extends in front of the bottom end portion of the central electrode, wherein the ground electrode is longitudinally defined by an inner surface, an outer surface and two lateral surfaces.
Spark plugs of the thus defined type are well known in prior art and are for example disclosed in U.S. Pat. No. 4,427,915, in U.S. Pat. No. 4,540,912 and in U.S. Pat. No. 4,568,855. The spark plugs of prior art, and in particular the spark plugs disclosed in the three mentioned U.S. patents, are generally provided with a ground electrode which has a rectangular cross section. Independently of the configuration of the central electrode the rectangular cross section of the ground electrode does not garantee a maximal resistance to the erosion of the ground electrode. In other words, in giving a different configuration to the cross section of the ground electrode it is possible, as will be explained hereunder, to improve the resistance to erosion of the ground electrode, i.e. to prolong its life period.